Fix modules_unittests on iOS.

Some of the module tests were failing on iOS, causing them to be ignored
on the trybots. Specifically this CL:
- Skips some tests that should not run on the simulator
- Fixes iOS file system test helpers
- Fixes failing nalu parser unittest

BUG=webrtc:4752
R=henrika@webrtc.org, peah@webrtc.org, sprang@webrtc.org
TBR=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/2352063002 .

Cr-Commit-Position: refs/heads/master@{#14472}

webrtc/modules/audio_processing/test/debug_dump_test.cc

diff --git a/webrtc/modules/audio_processing/test/debug_dump_test.cc b/webrtc/modules/audio_processing/test/debug_dump_test.cc
index b4e0ee7..ffaf609 100644
--- a/webrtc/modules/audio_processing/test/debug_dump_test.cc
+++ b/webrtc/modules/audio_processing/test/debug_dump_test.cc
@@ -516,8 +516,8 @@
   VerifyDebugDump(generator.dump_file_name());
 }
 
-#if defined(WEBRTC_ANDROID)
-// AGC may not be supported on Android.
+// AGC is not supported on Android or iOS.
+#if defined(WEBRTC_ANDROID) || defined(WEBRTC_IOS)
 #define MAYBE_ToggleAgc DISABLED_ToggleAgc
 #else
 #define MAYBE_ToggleAgc ToggleAgc